Functional control orthotics and accommodative devices known as "arch supports" are both worn in shoes, but there the similarity ends. Functional foot orthotics are distinctly different from those accommodative devices known as "arch supports." Arch supports are designed to cushion the foot. They are effective in reducing symptoms associated with flexible or fallen arches such as heel pain, plantar calluses, hammertoes and bunion deformities.
Functional control orthotics, in contrast, are prescribed orthoses that are form-fitted to a person's foot. They are designed to change the weight-bearing position of the subtalar joint of the foot. They are a medical device employed to support and align the foot and to improve the functions of the foot. They are designed to provide maximal and even distribution of the weight-bearing stresses over the entire sole of the foot.
The effect that poor foot mechanics has on the human body is now becoming better understood. As the foundation of a building supports the superstructure, the foot and ankle supports the body. If the building is unstable, it collapses. If the foot or ankle is unstable (overpronates), the joints above the foot are adversely effected. It has been said that walking is a unique activity during which the body, step-by-step, teeters on the edge of catastrophe. Man's bipedal mode of locomotion appears potentially catastrophic because only the rhythmic forward movement of the limbs keeps him from falling. The foot, being the base of support of the skeletal framework, plays an important role in gait. During early stance phase, the foot must be flexible so it can adapt to uneven ground surfaces. During late stance phase, it must be rigid to withstand the propulsive force generated by the big toe pushing off against the ground. Pronation and supination of the subtalar joint (see FIG. 10), the joint immediately below the ankle joint, gives the foot this dual capability. Pronation (FIG. 10C) of the subtalar join unlocks the foot (preparing it for heel contact), while supination (FIG. 10A) of the subtalar joint locks the foot (preparing it for toe-off).
Subtalar joint pronation has two important effects on the biomechanics of the foot: (a) it acts as a directional torque transmitter, absorbing the axial rotation of the leg and thus preventing it from entering the foot; and (b) it unlocks and prepares the forefoot for heel contact by diverging the axes of the midtarsal joint. One can easily demonstrate this shank to foot relationship by rotating the hips in a standing position. counter-clockwise rotation of the hips internally rotates the right leg and pronates the right foot (i.e. the foot rolls inward as the arch prolapses). From a causal point of view, pronation is a function of the pelvis, not the foot. The foregoing discussion presumes a normal functional relationship in which the range of pronation within the subtalar joint is dictated by pelvic rotation. However, an excessive range of foot pronation can result from structural weaknesses within the foot or shank. In such cases, the foot no longer follows the pronation pattern generated by the pelvis. This can lead to symptoms within the ankle, knee, hip and low back. A mechanical analogy is a bridge (the back) with an unstable foundation (pronated foot). In time everything above the unsound foundation shifts (soft tissue changes) and eventually collapses (joint changes). There appears to be a high correlation between excessive pronation and low back pain.
Functional orthotics are devices that control the range of subtalar joint motions and prevent excessive internal shank rotation (i.e. more that about 8 degrees of stance phase pronation). A functionally efficient orthotic must be fabricated around a neutral position foot replica (i.e. a positive foot cast). A neutral position foot replica is obtained by casting the patient in a nonweight-bearing position, holding the foot where the subtalar joint is neither supinated nor pronated, while the cast material hardens to produce a negative cast (i.e. a foot mold). Then the "positive" foot replica is cast from the "negative" mold. Using the foot replica, an orthotic is manufactured to make whatever adjustments the physician prescribes to accommodate the structural deficit.
For example, the topography presented in a forefoot varum deformity is illustrated in FIG. 11A, displaying that when the subtalar joint is held in its neutral position and the midtarsal joint is maximally dorsiflexed, the bottom (sole) of the forefoot is twisted inward (varum) relative to the posterior bisection of the heel bone (calcaneus). At midstance, forefoot varum introduces limb instability by decreasing the amount of foot-to-ground contact. In order for the medial plantar margin of the forefoot to reach the ground (a functionally stable relationship), the foot must roll excessively inward (e.g., excessively pronate).
This can be contrasted with the topography of a stable foot structure illustrated in FIG. 11B, (one which does not generate excessive foot pronation), displaying that when the subtalar joint is held in its neutral position and the midtarsal joint is maximally dorsiflexed, the bottom (sole) of the forefoot is perpendicular to the posterior bisection of the calcaneus (heel bone). This heel-to-forefoot relationship provides limb stability at midstance because the entire plantar surface of the foot contacts the ground.
An appropriate orthotic in this example as displayed in FIG. 11C eliminates medial instability of the FIG. 11A forefoot varum, by medially posting (wedging) the forefoot. This wedging increases the surface contact area between the forefoot and transverse plane by "building" the ground up to the foot; the pedal structure is now stable against the pull of gravity and excessive pronation does not occur.
Heretofore, the techniques available for manufacturing forefoot orthotics to stabilize varum and valgum deformities have been inadequate to the need. These techniques have not been capable of producing precise orthotics to meet physician's prescriptions. One could not predict whether a particular orthotic would be over, under, or exactly as prescribed. Moreover, as the effects of varum deformities are better understood, it is becoming increasingly necessary that orthotic manufacturing techniques permit incremental corrections so that the effects of the patient's varum or valgum deformities are gradually accommodated over time until a finally-prescribed orthotic structure is attained; thereby enabling the patient's body mechanics to be adjusted over a period of time.
Heretofore, orthotists have typically manufactured forefoot functional orthotics by taking a plaster cast foot replica (a "positive" cast) and modifying the bottom (sole) portion in an attempt to tailor that cast's sole portion to the prescribed forefoot posting. Then, the orthotic is grossly manufactured and then shaped to approximate the modified cast's sole portion to attain a final orthotic product that, when used by the patient, will properly align his/her subtalar joint. At best, the end product is artful inasmuch as the cast's sole-portion modification is inexact and the shaping done on the gross orthotic is also inexact. Once the foot replica's sole portion is modified, the orthotist no longer has an exact replica to use to check the accuracy of the orthotic that he is making. Consequently, the "posting", as rendered by the orthotist employing such a technique, can in actuality range, unpredictably, from several degrees too great to several degrees too little in relation to that actually prescribed by the physician. As a result, the exactness required to achieve incremental posting has not been achieved prior to the present invention.